(1) Field of the Invention
The present invention relates to improved membrane switch assemblies and specifically to membrane switch assemblies having internal cavities that are vented to the external environment in response to a pressure differential. More particularly, the present invention is directed to a technique for automatically compensating for pressure differences across flexible members which carry switch contacts and particularly members which in part define switches of an array of the type found in miniaturized keyboards. Accordingly, the general objects of the present invention are to provide novel and improved methods and articles of such character.
(2) Description of the Prior Art
Prior art membrane switch assemblies, such as switch arrays of the type employed in data entry keyboards, are typically constructed by laminating a spacer sheet between two printed circuit "boards", at least one of the "boards" being flexible. The substrate sheets of the printed circuits boards are positioned so that conductors thereon face each other. The switches are defined by providing the spacer sheet with apertures so that conductors of the spaced printed circuits can be urged into contact with each other. These prior art membrane switch assemblies were usually constructed so that the apertures in the spacer sheet formed cavities that were permanently sealed off from the surrounding environment. These cavities were usually filled with a fluid, primarily air.
The above-described prior art method of constructing membrane switch assemblies resulted in certain disadvantages. The major disadvantage, which was caused by hermetically isolating the interior of the cavities from the ambient atmosphere, was manifested when there was a change in the outside fluid pressure, for example the atmospheric pressure. If the machine which incorporates the membrane switch assembly was operated at an altitude where the atmospheric pressure is less than the cavity internal pressure, the greater internal pressure exerted an outward force upon the layers of the laminate comprising the printed circuit. The result of this outwardly directed force was a cushioning effect to the operation of the individual switches or keys. As the outside atmospheric pressure continues to drop the pressure within the cavities caused continued outward expansion and further interfered with the activation of the keys. In some situations the touch sensitivity may become so low that it will become difficult to determine by feel whether the key has been activated. In the extreme situation, with a very large differential between the outside atmospheric pressure and the pressure within the cavities, the membrane switch assembly may become distorted with some structural damage being caused by the outward expansion resulting from the pressure differential across the laminate walls.
A similar result occurs when the outside atmospheric pressure becomes greater than the pressure within the cavities. Such a reverse pressure differential will, for example, result when the mechanism incorporating the membrane switch assembly is located in a high pressure environment. Under these conditions the forces exerted upon the walls of the laminate will distort the walls of the laminate outwardly. In an extreme condition, when the pressure differential between the external pressure and the internal pressure becomes great, the switch may be activated by the walls of the laminate coming into contact with one another.
Another disadvantage in the construction of prior art membrane switch assemblies is apparent even under normal or expected operating conditions. The gas trapped within the cavities, being substantially non-compressible, provides resistance to compression of the walls of the laminate when a user trys to activate the keys. This results in a cushioning effect which is felt by the user of the membrane switch assembly. While under certain circumstances this may be a desirable effect, it may also reduce the users ability to either activate the key or "feel" that a switch closure has been achieved.
The prior art discloses methods which have been utilized to try to alleviate the above-discussed disadvantages of membrane switch assemblies. One such prior method involves incorporating internal passageways, within the spacer sheet, between the cavities. This allows displacement of the fluid medium, particularly air, between the internal cavities of the membrane switch assembly. When one key is activated the fluid within that cavity will be displaced by the downward movement of the membrane wall through the passageways into one or more other cavities. While the use of labyrith passageways will reduce the cushioning effect caused by the resistance to the downward deflection of the membrane wall of the trapped gas, it will not totally alleviate the problem. Further, problems resulting from a pressure differential between the outside atmospheric pressure and the internal switch assembly pressure were not solved by the use of labyrinth passageways. The pressure differential affects the passageways in the same fashion as it affects the individual isolated cavities.
A further technique for overcoming the above-discussed disadvantages suggested in the prior art involved permitting equalization of the internal fluid pressure with the external fluid pressure. This technique, in one form, requires providing a hole or plural holes in at least one of the outer layers of the membrane switch assembly. The hole or holes allows air, or any other fluid medium in which the switch assembly is operated, to flow freely into and out of the cavities. When the external pressure drops, gas will flow out of the cavities into the surrounding environment. When the situation is reversed, and the external atmospheric pressure is greater, gas flows into the cavities of the switch assembly. While this method solves the problems of the prior art membrane switch assemblies caused by differentials between the external and internal pressures, it created other significant disadvantages.
The major problem associated with providing a hole through an outer layer of the assembly is internal contamination. With air or other gas constantly flowing into and out of the membrane switch assembly, airborne contaminates are deposited within the assembly cavities. These airborne contaminates include dust, water vapor and airborne chemical contaminates. These contaminates cause deterioration of the switch assemblies particularly the conductive circuits. Water vapor, for example, will cause oxidation of the conductive circuits. It may also be possible that the walls of the assembly will deteriorate, depending upon the type of chemical contaminate present. This deterioration of the assembly shortens its lifespan and thus results in additional operating cost incident to repair or replacement.
A basic problem with prior art membrane switch assemblies is thus the need to provide means to stabilize and equalize the pressure differential across the printed circuit boards that carry the moveable switch contacts without allowing deterioration of the switch assembly by airborne contaminates. It is also recognized that any method devised to solve this problem will have to be characterized by reliability and minimum added production cost. If the solution to the problem resulted in a more cumbersome and expensive switch assembly, the usefulness of the membrane switch assembly would be impaired.
Accordingly, the general objects of the present invention are to overcome the above-discussed and other disadvantages of the prior art without reducing the usefulness or economic attractiveness of the membrane switch assembly.